Fine powders composed of silicone elastomers already find use as additives in cosmetics and toiletries, for example as a matting agent, as an absorber for sebum or for generating a silky skinfeel, as additives for improving the mechanical properties of polymers and lacquers or coating materials, for example for increasing abrasion or scratch resistance and also impact resistance, and also as antiblocking agents for improving the lubricant properties of a wide variety of different surfaces, as flow or dispersing aids in powders, as additives in toners, and as a mild abrasive in washing and care formulations.
Several methods are known for the preparation of such particles. In principle, irregularly shaped silicone elastomer particles can be obtained by grinding operations of a particular bulk elastomer, but spheroidal or spherical particles generally offer performance advantages, in particular when attractive tactile properties of the particle-additized minerals and formulations are desired. Typically, such particles are prepared by crosslinking reactions within reactant droplets or growth/application of a polymer on to a particle core. Crosslinking reactions may be hydrosilylation reactions, condensation reactions, dehydrogenative coupling reactions or free-radical polymerizations.
Silicone particles from hydrosilylations are described, for example, in U.S. Pat. No. 4,761,454, JP 2003301047 and EP 1 074 575, hydrolysis and condensation reactions for preparing silicone particles can be found in EP 1 130 046, WO 2006/016968, JP 2003002973, U.S. Pat. No. 6,753,399, EP 0 765 896 and EP 0 744 432, whereas U.S. Patent Application Publication No. 2004/0156808 describes a dehydrogenative coupling reaction for this purpose. Finally, DE 10 2004 053 314 describes copolymers obtainable by means of free-radical polymerizations.
Miniemulsion polymerization of silicone acrylates to give nanoscale particles using a conventional emulsifier and a common molecular free-radical initiator, for example AIBN, is known from the literature (“Polydimethyl Siloxane Latexes and Copolymers by Polymerization and Polyaddition in Miniemulsion”, Katharina Landfester, Ute Pawelzik, Markus Antonietti, Polymer, 46 (2005), 9892-9898). However, the process described in the prior art does not afford microscale particles which possess the desired performance properties, for example such particles cannot achieve good skinfeel which is desired for personal care applications.
Aqueous emulsions stabilized in the solid state were described in 1907 by S. U. Pickering (“Emulsions”, Spencer Umfreville Pickering, Journal of the Chemical Society, Transactions (1907), 91, 2001-2021) and are considered to be particularly stable against coalescence. For example, DE 10 2004 014 704 describes the preparation of emulsions stabilized with pyrogenic particles. A good overview of the properties of such stabilizing solid particles can be found in “Particles as surfactants—similarities and differences” by Bernhard P. Binks (Current opinion in colloid & interface science, 7 (2002), 21-41). The prior art also includes so-called “Janus particles”, amphiphilic particles with a hemispherically modified surface, as described, for example, in FR 2 808 704. Particularly suitable particles for emulsion stabilization are nanoscale, predominantly inorganic particles, for example silica particles, which are commercially available as “LUDOX®” in the form of aqueous sols and dispersions from Grace Davison. U.S. Pat. No. 3,615,972 describes the use of LUDOX® particles for emulsion stabilization of methyl methacrylate with subsequent polymerization. The mechanism discussed in the literature for the stabilizing action is the agglomeration of the particles and the enrichment of the agglomerates at the water/oil interface (“The mechanism of emulsion stabilization by small silica (LUDOX®) particles”, Helen Hassander, Beatrice Johansson, Bertil Törnell, Colloids and Surfaces, 40, (1989), 93-105).
The suspension polymerization of Pickering emulsions of sparingly water-soluble or water-insoluble reactants must, according to the present state of the art, be started by means of a free-radical initiator dissolved in the oil phase; the use of water-soluble free-radical initiators, for example with styrene as the sole monomer, leads to incomplete reaction and coagulation (“Pickering stabilized miniemulsion polymerization: Preparation of clay armoured latexes”, Severine Cauvin, Patrick J. Colver, and Stefan A. F. Bon, Macromolecules 2005, 38, 7887-7889). A disadvantage of a suspension polymerization that is initiated with a molecular free-radical initiator is that reaction products of the free-radical initiator remain in the polymer and can become perceptible, for example, through odor nuisance or else through irritant or toxic properties.
The prior art processes described for preparing silicone particles include hydrosilylation, free-radical polymerization, dehydrogenative coupling or condensation of emulsified precursors, spray processes, and the injection of the precursors into a suitable media with subsequent immediate crosslinking.
The particles thus prepared predominantly have the disadvantage that they are not obtained as a free-flowing powder and are therefore difficult to handle, i.e., for example, difficult to dose, and are homogenizable in the particular formulations only with a high level of complexity. In addition, the particles prepared in the prior art usually contain proportions of crosslinking catalysts, often including elements of transition group 8 of the Periodic Table of the Elements, emulsifiers and possibly further processing aids. In cosmetic formulations, and also cleaning and care products, this is undesired or at least problematic.
A further disadvantage of the particles prepared according to the prior art is that polydimethylsiloxane-like particle surfaces can be modified only with difficulty.
However, such modification is often desired in order to be able to adapt the particles to the different technical requirements, i.e., for example, to enable their attachment to various matrices or to facilitate or actually make possible processability into formulations.
Some of these disadvantages can be overcome by composite particles. Composite particles refer here to core-shell particles, and particles into which additional solids have been incorporated.
For example, U.S. Pat. No. 4,946,893 (EP 0 319 828) describes the preparation of silicone particles filled with inorganic particles by means of a hydrosilylation reaction in aqueous phase, and U.S. Pat. No. 5,176,960 describes the preparation of highly filled, mechanically durable silicone particles by means of mixing hydrophobized SiO2 with a diorganopolysiloxane and subsequent curing by spray-drying.
In contrast, core-shell particles allow modifications, in some cases controlled, of surface properties, which influence the desired performance properties.
According to the preparation process and use of the core-shell particles, their particle size may be within the nanometer or micrometer range. Core-shell particles can be prepared by literature processes; for instance, EP 0 661 334 describes silicone particles surface-coated with an organopolysilsesquioxane resin and the preparation thereof, U.S. Patent Application Publication No. 2006/0084758 describes the preparation of silicone particles surface-modified with smaller silicone particles, and silicone particles coated subsequently with SiO2 from the aqueous phase can be found in EP 0 079 322, and EP 0 516 057. In addition, EP 0 079 322 describes silicone particles surface-coated with SiO2 with the aid of an oily phase. Core-shell particles with a silicone polymer core and organopolymer shell are described in DE 10 2004 047 708 and DE 10 2004 022 406 (use in aqueous coating materials in EP 0 882 105, and in powder coatings in EP 0 852 610).
Moreover, there are numerous documents which relate to core-shell structures with an inorganic core and silicone shell, for example EP 0 822 232 and PP 0 433 727.
A disadvantage of these prior art processes for obtaining core-shell particles is that they are time-consuming and energy-intensive, multistage processes.